From the prior art, transmissions are known, which comprise rotational speed detection devices for generating a speed signal, for a motor vehicle. These rotational speed detection devices comprise in essence a signal emitter or speed indicator connected rotationally fixed to a corresponding transmission shaft, this being associated with a fixed speed sensor in order to measure the rotational speed from the speed indicator.
In known motor vehicle transmissions with two countershafts, a main shaft is mounted with sufficient play between a drive input and a drive output shaft, in such manner that during operation under load the main shaft is automatically centered. Owing to the play provided, in the load-free condition and while rotating the main shaft can undergo a certain amount of movement and may move away from its nominal position both axially and radially. The types of movement involved here range from radial flutter to a so-termed wobbling motion in which the ends of the main shaft move in opposite directions. From the prior art rotational speed measurement takes place by virtue of a toothed disk connected fixed on the main shaft and a sensor, which can be arranged radially or axially on the toothed disk. A disadvantage of this is that if the main shaft is wobbling, the toothed disk fixed on the main shaft also follows this wobbling motion. Since such a toothed disk has a relatively large diameter, the movements of the toothed disk produced by the wobbling motion are also correspondingly ample. However, for measurement technology reasons the toothed disk must only move within a very narrow tolerance range. If it moves outside this tolerance range exact rotational speed measurement can no longer be guaranteed, because the teeth of the toothed disk or the gaps between them can no longer be detected reliably by the sensor. Furthermore, in such a case a ‘smart’ sensor has to be used, which will emit a correct signal despite the wobbling motion of the main shaft. Such a sensor, however, is substantially more expensive than a conventional rotational speed sensor with no additional evaluation electronics. Besides, the structural length of the transmission is increased by the thickness of the toothed disk.
In the previously unpublished application with file number 10 2006 023 554 by the present applicant, an arrangement for determining the rotational speed of a transmission shaft is disclosed. This arrangement comprises a speed indicator connected in a rotationally fixed manner to the transmission shaft, and a fixed speed sensor by which the speed can be measured from the speed indicator. The speed indicator is provided at the circumference of an axially movable sliding sleeve arranged in a rotationally fixed manner on the transmission shaft. The speed sensor is arranged on a long sensor arm and is positioned on the sliding sleeve in such a manner that the rotational speed can be determined from the speed indicator in the radial direction.
A disadvantage of this is that to position the speed sensor a long, freely suspended sensor arm is needed. The longer such a sensor arm is, the greater is the risk that an external vibration excitation will cause the arm itself to vibrate. Such vibration can result in an erroneous speed signal. In this case too, therefore, a ‘smart’ sensor has to be used, which will deliver a correct signal despite the wobbling motion of the main shaft. To avoid clashes during a shifting operation the sensor arm must be able to enter a shift fork opening, so the shift fork arms must be designed asymmetrically and the shift fork cannot be made too massive.